Cal here from Shopify, thanks for reaching out to us. You won't be able to close your store manually without settling the existing charges on the account. When your billing cycle comes due, there will be three attempts to charge what you owe Shopify. If we are unable to charge you after those three attempts, your store will be frozen until you pay that sum.
You should look to settle those charges if you are looking to close down your store. If you want to learn more about billing on Shopify or about specific charges we have a great help center document here.